The Z-scheme heterojunction design can not only minimize photogenerated electron-hole pair recombination but also ensure strong redox abilities for driving photocatalytic reactions. Hence, novel P-g-C3N4/α-Bi2O3 binary nanocomposites were successfully fabricated by a combined hydrothermal-calcination method. The as-synthesized samples were systematically characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy/energy-dispersive X-ray spectroscopy, transmission electron microscopy, ultraviolet–visible spectroscopy, photoluminescence (PL) spectroscopy, N2 isotherm measurement, electrochemical impedance spectroscopy (EIS), thermogravimetric analysis, and electron paramagnetic resonance. The photocatalytic efficiency of the as-prepared samples was assessed via decontamination of refractory and endocrine-disrupting benzophenones (BPs) under visible light irradiation. The 25 wt% P-g-C3N4/α-Bi2O3 nanocomposite exhibited higher photocatalytic activity for 4-hydroxybenzophenone (4H-BP) and benzophenone-1 degradation than P-g-C3N4 and α-Bi2O3. The apparent enhancement of the photocatalytic activity might be mainly attributed to the enhanced light-harvesting ability and the heterojunction formation, which significantly facilitates photogenerated charge carrier separation and transfer, as confirmed by the PL and EIS findings. Notably, 25 wt% P-g-C3N4/α-Bi2O3 exhibited robust reusability and stability with high efficiency in 4H-BP removal after four repeated photocatalytic cycles.